Centralized traffic controlling system



27, 195?; F. x. REES CENTRALIZED TRAFFIC CONTROLLING SYSTEM 7 Sheeis-Sheet l Original Fi led Nov.

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CENTRALIZED TRAFFIC CONTROLLING SYSTEM Original Filed Nov; 12, 1947 7 Sheerls-Sheet '7 MODULATOR TRANSMITTER fi conmox. RELAY or? swncu R- HC2-8. TYPICAL TONE FILTER I (B+) a cz TRGGER CONTROLLED RELAY RECEWER g AMPLlFlER-l GRCUT i 1.1 ca i 2 TONES MODULATING CARRlER F1 FOR CONTROLS TONE N0. USE

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TONES MODULA'HNG CARRIER FE FOR INDICATIONS USE ICE Zhmentor (Ittorneg Patented Nov. 27, 1951 CENTRALIZED TRAFFIC CONTROLLING SYSTEM Frank X. Rees, Chili, N. Y., assignor to General Railway Signal Company, Rochester, N. Y.

Original application November 12, 1947, Serial No. 785,327. Divided and this application August 15, 1950, Serial No. 179,443

7 Claims. (Cl. 177-353) This invention relates to centralized traffic control systems for railroads, and it more particularly pertains to centralized traific control systems in Which communication between a control ofilce and the respective field stations is maintained by transmission of distinctive frequency currents or tones.

This application is a division of my copending application Ser. No. 735,327, filed November 12, 1947, now Patent No. 2,543,869, dated March 6, 1951.

Problems relative to interference between a plurality of transmitters in the communication part of a centralized traflic control system, conveniently called a CTC system, are solved in some instances by providing different communication channels for the respective control office and field station transmitters, and in other instances by various lock-out organizations wherein only one station is allowed to transmit at atime.

An object of the present invention is to solve the problem of lock-out between indication station transmitters by use of a link transmission organization wherein the indications are relayed from station tostation, and no station can transmit its indications by the distinctive rnodulation of an indication pulse until it receives a so called permission pulse from the most distant station, such pulse being characterized byhaving only a station call tone applied thereto. In this way a system of superiority is set up wherein the next distant station is always superior with respect to the transmission of indications.

Another object of the present invention is to transmit pulses continuously at a predetermined rate at the most distant field station and to relay such pulses by link communication through the respective field stations whereby the pulses can be selectively modulated byany field station as a means for communicating indications from that field station to the control omce provided that there have been no indication modulations applied to a corresponding pulse by a more distant station.

Another object of theplesent invention is to transmit but one indication per pulse from any field station, such indication being identified because of a distinctive combination of a plurality of tones modulating the pulse simultaneously.

Another object relative to the communipation of indications is to transmit from therespective field stations only indications of new changesin the positions of devices as compared to the retr n iss o o o d ind at ns a ha been tr n mit ed be re Thu a om ete c eck o al of the indication a a ,fi l station is not o dinarily transmitted each time there is a change in a device to be indicated, but it is provided that a complete recheck of indications can be efiected in response to the transmission of a recheck control from the control oflice for that particular field station. Under such recheck conditions, indications of all devices are transmitted from that field station by successive pulses, one indication per pulse.

Another object of the present invention is to accomplish the above objects by radio link communication from the most distant indication station through intermediate indication stations to the control ofiice.

Another object with respect to the radio communication of indications is to communicate indications by radio from respective field stations to the control omce on a single assigned carrier frequency for all indication station transmitters, all without interference due to a radio receiver hearing a plurality of transmitters at the same time. This is accomplished by suppressing reception at a field station during transmission by all field stations within transmitting range that are closer to the control office.

Another object of the present invention is to communicate controls from the control oifice to field stations for the control of switches and signals and the iike by distinctive modulations applied to a carrier Wave or to a line circuit in accordance with the requirements of practice, the modulations of the carrier wave or line circuit being normally ineffective but being rendered effective in response to manual designation of a particular field station to which controls are to be transmitted.

Other objects, purposes, and characteristic features of the present invention Will be in part obvious from the accompanying drawings, and in part pointed out as the description progresses.

In describing the invention in detail reference is made to the accompanying drawings in which similar letter reference characters are used to designate similar parts of the organization, such parts being generally made distinctive as to the respective field stations with which they are associated by use of preceding numerals or by the use of exponents characteristic of the respective field stations; and in which:

Fig. 1 illustrates control ofiice apparatus for the transmission of control pulses;

Fig. 2 illustrates the manner in which track switches and signals at respective field stations are controlled in accordance with control pulses received from the control office;

Fig. 3 illustrates the organization of apparatus 3 at the control office for the control of indicator lamps in accordance with indications received from the field stations;

Fig. 4 illustrates the organization of the apparatus at the intermediate field station No. 2 for the communication of indications to the control ofiice;

Fig. 5 illustrates the organization of apparatus" at the intermediate field station No. 3 for the communication of indications to the field station No. 2;

Fig. 6 illustrates the organization of apparatus at the end field station No. 4 for the transmission of indications to field station No. 3;

' Fig. '7 illustrates a typical tone generator such as is used for the modulation of transmitted carrier wave pulses;

Fig. 8 illustrates a typical tone filter used in the demodulation of pulses that are received at the respective field stations and at the control oflice;

Fig. 9 is'a tabulation showing the manner in which distinctive tones are employed for the communication of controls in this embodiment of the present invention; and,

Fig. 10 illustrates the manner in which distinctive tones are assigned for use in communication of respective indications from the various field stations to the control office.

For the purpose of facilitating the disclosure of the present invention as to its mode of operation, schematic wiring diagrams have been used to illustrate the circuit organization rather than attempting to show the specific structure and arrangement of parts that would be employed in practice. Rather than to show wiring connections to all sources of energy, the symbols and have been used to indicate connections to the respective positive and negative terminals "of suitable batteries, or other sources of direct current; the symbols 13+ and B have been used to indicate connections to the respective positive and negative terminals of a suitable plate or B power supply the terminal B being connected to ground; and the symbols BX and CX have been used to indicate connections to the respective terminals of a suitable alternating current power supply.

CONTROL COMMUNICATION APPARATUS With reference to Fig. 1, control oifice apparatus for the transmission of controls is illustrated wherein a miniature diagram of the track layout for which the CTC system is provided is generally constructed on a control panel and suitable indicator lamps are associated with respective portions of the track diagram to indicate occupancy by trains of respective portions of the trackway, and for other purposes well known to those skilled in the art.

Suitable levers, or switches, are provided on the control panel at the control office for designation of the respective controls to be transmitted to the field stations for governing track switches and signals, maintainers call signals, and indication recheck controls according to the requirements of practice. It is thus illustrated that the signal control lever 2SGL is provided on the panel of the control machine at the control oiiice for governing the signals at field station No. 2 which is located at the A end of the passing siding of this embodiment of the present invention, and the lever ZSML is provided for governing the operation of the track switch 2SW at the same field station. The levers 2MCL and ZRCL are used in governing the transmission of control codes for respective maintainers call and recheck purposes. The control levers 3SGL, 3SML, 3MCL and 3RCL are associated with the designation of controls for communication to field station No. 3 for governing the signals, track switch, maintainers call signal and recheck controls at that station.

In accordance with the usual practice in designating controls for communication to respective field stations in a CTC system, a start button SPB is provided on the panel of the control machine for each field station for rendering effective the transmission of controls to that field station as selected by the above mentioned control levers.

In accordance with the initiation of the transmission of controlsf or field station No. 2 by the actuation of the start button 2SPB, a relay 2-30 is picked up, and the picking up of that relay causes the respective inputs of a plurality of tone generators to be governed in accordance with selections of the control levers associated with the communication of controls to field station No. 2.

It is thus provided that in response to the initiation of the transmission of a control pulse by the actuation of a button SPB, certain of the tone generators are rendered effective to apply their outputs simultaneously to a modulator, and the output of the modulator is applied to a CW transmitter Fl for the modulation of the carrier wave of that transmitter, the duration of the modulation being governed by the length of the time that the push button SPB is depressed. The output-of the CW transmitter Fl is supplied to the antenna Al for radiation to the respective field stations Nos. 2 and 3 as illustrated in Fig. 2, which are both assumed to be within range of the control ofice transmitter.

With reference to Fig. 2, a track layout is illustrated having a passing siding and having field station No. 2 at one end of the siding and field station No. 3 at the other end. The passing siding is connected to the main track at the lefthand end by the track switch 2SW, and at the right-hand end of the track switch 38W. The switch machines ZSM and 3SM provide for the power operation of the track switches 2SW and 3SW respectively in accordance with switch controls communicated from the control office by the OTC system.

The signals 2RA and 2R3 are provided for governing traflic to the right through the track switch 2SW, and the signals 2LA and ZLB are provided for governing trafiic to the left through the track switch 2SW. Similarly the signals 3LA and 3LB are provided for governing traffic to the left through the track switch SSW and the signals 3RA and 3RB are provided for governing traffic to the right through the track switch 3SW. The signals are illustrated as being of the search light type such as is shown for example in the patent to O. S. Field No. 2,239,316, dated April 22, 1941, but it is to be understood that other types of signals such as semaphore, position light, or light signals having individual color light units can be as well employed in accordance with the requirements of practice.

Each of the field stations is provided with a receiver Fl, and such receiver is tuned to the frequency Fl which is transmitted from the control office. The demodulated output to each of the receivers is connected as illustrated to a bank of tone filters, the bank of tone' filters at each field station comprising filters tuned to respective difierent frequencies. Each filter includes amplifier, trigger circuit, and-control tube stages as is shown by the typical tone filter il-.- lustrated in Fig. 8. It is to be understood that a larger or smaller number of tone filters is to be provided as required in practice in accordance with the number of field stations included in the CTC system, and the number of distinctive controls to be communicated to each field station.

A station relay SC is provided at each field station for governing the response of the switch and signal control communication apparatus at that field station. Such relay SC is responsive .to the output of a particular tone filter corresponding to the station call tone for that field station.

A switch control relay W2 is provided at each of the field stations for governing the operation of the track switch SW at that field station. According to Fig. 2 the switch control relay WZ at each field station is of the magnetic stick type so as to maintain its last operated position, and it is governed by the output of the tone filter Nos. 4 and 5, subject of course to the energization of the station relay SC at that field station.

The relays RGZ and LGZ and B at each of the field stations are associated with the control of the signals at that station, the relay RGZ being associated with the clearing of signals governing traific to the right, and the relay LGZ being associated with the clearing of signals governing traffic to the left. The relay B is associated with the restoration to stop of the signals at its associated field station in accordance with a stop control communicated from the control ofiice, and the OS track relay TR at each field station is effective to automatically restore the signals to stop upon the passage of a train. The relays RGZ, LGZ and B are governed by the output of tone filters Nos. 6, '7 and 8 respectively.

The output of the tone filter No. 9 is used in the control of a maintainers call relay M0 at each of the field stations, and such relay MC governs suitable indication means such as an indicator lamp MK which can be extinguished when the maintainers call is acknowledged by the actuation of a suitable cancel button or switch MPB.

A relay RC is associated with the output of tone filter No. 1 at each of the field stations for the purpose of initiating the transmission of a recheck of all indications at the associated field station in accordance with a .distinctive control transmitted from the control omce.

INDICATION COMMUNICATION APPARATUS With reference to Fig. 6, the indication communication apparatus for the most distant field station of the system is illustrated wherein the oscillator l fiiiC'l" is normally active to initiate the communication of indication pulses. It is to be understood that any suitable pulse forming means may be employed but for this embodiment of the present invention it is assumed that the pulses are formed by a suitable oscillator such as is disclosed, for example, in the patent to O. S. Field, No. 2,351,588, dated June 20, 1944. The oscillator l86CT is assumed to generate 180 pulses per minute with oil and on periods of substantially the same duration. The oscillator is so constructed so as to have contacts closed during the on period which is illustrated as an upper dotted position and contacts closed during the "01? period as illustrated by the dotted downward position.

For the purpose of initiating the communication of indications from the end field station No.

=4 as illustrated in Fig. 6 a change relay OH is associated with each'device to be, indicated at the control oifice, and such relay is maintained normally energized by a stick circuit which is opened in response to a change in its associated device. Associated with each change relay CH is a repeater relay CHP, the contacts of which actually govern the frequencies at which transmitted pulses are modulated.

A bank of tone generators is provided at field station No. 4 as illustrated in Fig. 6 for generating the frequency currents used for modulation of the carrier output of a suitable transmitter F2. Each of the tone generators includes a control tube such as the tube Tl shown as partof the typical tone generator illustrated in Fig. 7.

It is thus provided that the input to the respective tone generators is selected by the relays CHP, together with their associated devices to have their positions indicated, and applied to the tone generators in the form of pulses generated by the oscillator I8OCT and in turn, the output frequency current of the tone generators is applied in the form of pulses through the modulator to the transmitter F2 for modulation of the carrier wave radiated by the antenna A4. It is thus provided that the transmitter F2 output as applied to the antenna A4 is at a constant carrier frequency F2, and such output is modulated by one or more tones during on periods comparable to the on periods formed by the oscillator l8flCT The apparatus associated with the communication of indications to the next station nearer the control office from each of the intermediate field stations (see Figs. 5 and 6) is comparable to that described for the end field station No. 4 except that the oscillator [800T at each of such intermediate field stations is normally inactive, and the respective off and on periods governing the transmission of modulations from that intermediate field station are governed in response to the reception of pulses from the next distant field station. Thus the receiving antenna A5 of Fig. 5 receives pulses from the transmitter F2 of the field station No. 4 (see Fig. 6), and applies such pulses to the receiver F2 .(see Fig. 5) at field station No. 3.

A bank of tone filters is provided at field station No. 3 as illustrated in Fig. 5 to be used in rendering a bank of relays R. distinctively responsive to the tone modulations of the pulses received from the next distant field station. The relays R are used in relaying indications through the field station with which they are associated from the next distant field station to the next field station nearer the control ofiice.

The receiver relays ERPI, BRPB and ZRPB at station No. 3 are energized in response to pulses received at field station No. 3 and these relays are provided with slow drop away characteristics in accordance with shunting resistor and rectifier circuit combinations in order to provide desired timing operations in a manner and for purposes to be more readily apparent as the description progresses.

The relay Y is used in allowing the modulation of the transmitter F2 at field station No. 3 in accordance with indications to be transmitted from that field station only at times when there are no indications to be relayed through from the next distant field station. There is therefore a bank of tone filters provided at field station No. 3 which are selectively rendered active to modulate the transmitter F2 at that station either in accordance with indications to be transinitted from field station No 3, or in accordance with indicationsrelayed. through field station No. 3 from a more distant field station. The output frequency currents of the tone generators is fed through a suitable modulator to the input of the transmitter F2, and the antenna A6 radiates the carrier frequency F2 (the same for each field station) which is selectively modulated by one or more tones. The apparatus thus described as being employed at the intermediate field station No. 3 for the communication of indications is to be considered as typical of the indication communication apparatus provided at field station No. 2 or at other field stations as may be required in practice, such apparatus differing only 'in minor respects in each of intermediate stations because of the difierent tones used for station call purposes in a manner to be hereinafter more specifically considered.

a With reference to Fig. 3, the apparatus at the control ofiice for the reception of indications is illustrated wherein the antenna A! applies to a suitable receiver F2 that which is transmitted from the nearest field station for the communication of indications. The receiver F2 is tuned to the frequency F2 used for the communication of indications, and such receiver has a demodulated output which is fed to the input of a bank of tone filters used in governing the selective energization of a group of relays B, one relay R being provided to be responsive to the output of each tone filter. A bank of magnetic-stick relays is provided as illustrated in Fig. 3, together with associated indicator lamps, and these magnetic stick relays are selectively positioned to govern the energization of the respective indicator lamps in accordance with the conditioning of the relays R during each pulse received from the nearest field station.

Having thus considered the general organization of the system according to this embodiment of the present invention, the circuit organization will be hereinafter more specifically described together with the mode of operation of the system under certain typical operating conditions to be encountered in practice.

OPERATION Use of tones The tones used in this embodiment of the present invention are preferable within the audio range of frequencies, although it is to be understood that other frequency ranges can be employed such, for example, as the use of higher frequencies where it is desired to employ crystal filters and crystal controlled frequency generators as is disclosed, for example, in the application of Donald Blaisdell, Ser. No. 758,833, dated July 3, 1947, now Patent No. 2,559,390 dated July 3, 1951.

A suitable range of frequencies for tones used in this embodiment of the present invention is found between 3,000 and 5,500 cycles, and it is found that where LC filtering means is employed, a separation of 300 cycles between'tones is sufiicient to prevent interference between respective tone channels. Care must of course be exercised to allocate tone frequencies within a band where harmonics cannot cause erroneous controls or indications.

It will be apparent that a sufiicient number of distinctive tone frequencies can be allocated withinthe band from 3,000 to 5,500 cycles to allow the assignment of a distinctive tone for the station call of each of aslargea number of field stations as may be employed in the CTC system, there being only a small number of distinctive tones required for the communication of the respective switch and signal and maintainers call and recheck controls to any particular station which is called. For the purpose of simplification of the present disclosure, however, the number of distinctive tones employed for both the communication of controls and the communication of indications has been maintained at a minimum rather than unnecessarily complicating the present disclosure by employing a larger number of tones than are actually required for a simple embodiment which is particularly adapted to disclose the mode of operation of the system and the manner in which the system can be expanded to meet the requirements of practice.

With reference to Fig. 9, a table is shown'of tones used for modulating the control carrier frequency Fi for the transmission of controls from the control office, each tone being identified by a number indicative of a distinctive assigned, tone frequency. According to this table the tones Nos. 2 and 3 are provided for calling the field stations Nos. 2 and 3 respectively, as it is assumed that for this embodiment of the present invention the field stations Nos. 2 and 3 are the only control stations (see Fig. 2) employed. It is to be understood, however, that if a larger number of stations is employed, a distinctive tone is assigned to the station call for each of such larger number of control stations.

The tones Nos. 4, 5, 6, 7 and 8 are used in the transmission of switch and signal controls from the control office, and the tones Nos. 1 and 9 are used in the transmission of recheck and maintainers call controls respectively.

With reference to Fig. 10, a table of the tones is illustrated which are used in modulating a carrier frequency wave F2 for the communication of indications. Similar to the allocation of tones for the communication of controls, a distinctive tone is allocated for the call of each indication field station, the stations being called successively,-

however, by link transmission from the more distant stations. It will be noted that the station call tones assigned to the respective stations for station call are also used by these stations for station registration at the control office to identify the stations transmitting indication tones. As a matter of convenience the tone numbers for station call and station registration have been made to correspond to the number of the indication field stations to which they are assigned. Thus tone No. 2 is used for registration at the control ofiice for station No. 2, tone No. 3 is used for registration of indication station No. 3, and tone No. 4 is used for registration of indication station No. 4. Additional field stations will of course require the assignment of additional distinctive tones for registration and call purposes;

It is further provided that these indication station tones serve a dual purpose in that they are also used for station calling. The indication station call tones are used in link communication wherein each time station No. 3 is to be called from station No. 4, the tone No. 3 is used as a station call tone, and similarly the calling of station No, 2 from station No. 3 is accomplished by use of the station call tone No. 2. The tone No. 1 is reserved for the calling of the control OfiiCG by the nearest indication field station which 9 is assumed to be indication field station No. 2. Although it will be readily apparent that a single distinctive tone can be used for communication of each of the indications at a given indication field station to the control office, as a matter of economy in the number of distinctive tone filters and generators required, and for other considerations, it is provided that a distinctive combination of three tones simultaneously transmitted is used for the communication of each indication. Thus, for example, the tones Nos. 5, 6 and 7 are simultaneously transmitted for indicating a signal clear for governing trafiic to the left, and the combination of tones Nos. 5, 6 and 8 is used for an indication of a signal clear for governing traific to the right.

Normal conditions Normally the system is inactive for the communication of controls, and it is rendered active only in response to the manual actuation of a start button "SPB (see Fig. 1). The trans mission of a carrier wave at the frequency Fl, is however rendered constantly effective by the oflice transmitter Fl, although it is to be understood that means may be provided for the purpose of saving power, and for other considerations, to permit the radiation of energy from the the antenna Al only during the transmission of a control modulated pulse.

At the field stations (see Fig. 2) the track switches Z'SW and 38W are assumed to be normally in alignment for through traflic movements on the main track, and the signals governing traific through such track switches are normally at stop in accordance with the usual practice in signaling organizations of this character.

The reception at the field stations by the antennas A2 and A3 (see Fig. 2) of the carrier wave applies an input to the receivers Fl at the respective field stations, but normally there is no demodulated output of the receivers because the carrier wave received at the frequency Fl normally has no modulations, as modulations are applied only in response to the actuation of a start button SPB at the control office for the transmission of a control pulse.

Each of the change relays CH at the field stations (see Figs. 4-6) is maintained normally picked up by its stick circuit which in addition to extending through contacts of the associated devices to be indicated extends through a contact of the recheck relay RC at that station. The relay 3CH for example (see Fig. 6) is associated with the transmission of track occupancy indications in accordance with the position of the track relay TR. which is assumed to be governed by the OS track section at field station No. 4 comparable to the manner in which the track rel'ayZTR, is indicated as being associated with OS track section 2T at field station No. 2. Thus the stick circuit for the relay 3CH extends from including front contact Ill of relay 3CH winding of relay 3CH contact H of the track relay TR, and back contact 12 of the recheck relay RC to Similarly the relay 2CH which is associated with the transmission of switch indications is normally maintained picked up by its stick circuit which extends from including front contact l3 of the relay 2CH winding of relay 201-1 contact I4 of the normal switch correspondence repeater relay NWC, contact l5 of the reverse correspondence switch repeater relay RWC and back contact l2 of relay RC to The relay ICH is associated with a change in the signals at field station No. 4, and thus its stick circuit is dependent upon the signal stop repeater relay contacts i6 and H. Such stick circuit extends from including front contact [8 of relay I OH, winding of relay iCH contact I6 of relay LM contact I"! of relay RM and back contact I 2 of the recheck relay RC to In accordance with the normal energization of each change relay CH at field station No. 4, the associated relay CHP are also maintained picked up because of the energization of its stick circuit which is dependent upon the associated relay CH Thus the relay 3CHP is maintained picked up by a stick circuit extending from (-1-), including front contact is of relay 3CH front contact '20 of relay SCHP' and winding of relay 3CI-IP to In a similar manner the relay ZCHP is maintained picked up by a stick circuit in accordance with closure of front contact 2| of relay 20H, and the relay ICHP is maintained picked up in accordance with closure of front contact 22 of relay iCH Wi h all of the relays CHP picked up, the indication control relay I is maintained picked up in accordance with there being no indications to transmit. The relay I is energized under such conditions by a circuit extending from including front contact 23 of relay SCI-I1 front contact 24 of relay 2CHP front contact 25 of relay ICI-IP and winding of relay I to It is thus provided that the dropping away of any relay CHP because of the dropping away of its associated relay CH is effective to cause the dropping away of relay I and thereby render the modulation of the next pulse to be transmitted efi ective in a manner to be hereinafter considered. Having thus considered the circuits by which the relays CH and CHP at field station No. 4, and the relay I at that station, are maintained picked up, it is to be understood that similar relays at other field stations, such, for example, as at field stations Nos. 2 and 3 as illustrated in Figs. 4 and 5 respectively are maintained picked up by the energization of similar stick circuits.

With reference to Fig. 6, each time that an on period is created during normal conditions at a time when there are no indications to be transmitted, the relay FF is picked up by the energization of an obvious circuit closed at contact 26 of oscillator ISHCT in its upper position, and in accordance with the picking up of that relay, the closure of its front contact 27 conditions a circuit whereby the shifting of 0scillator contact to its lower position causes radiation from the antenna A4 to be rendered effective, and the carrier wave radiated is modulated by the frequency of the tone generator No. 3.

The tone generator No. 3 of Fig. 6 is rendered effective to apply its output to the modulator because of the connection of ground to shunt the bias of a control tube governing the output of the tone generator No. 3. Such ground connection includes contact 28 of oscillator l8GCT in its lower position and front contact 29 of relay F? to the tone generator No. 3. Inasmuch as tone generators are well known to those familiar with the art, it is believed unnecessary to show specifically the details of such generators and their control tubes, but, for the purpose of facili- I I! tating the understanding of the present invention, a typical tone generator which may be employed is illustrated more specifically in Fig. '7 wherein the oscillator generates the particular tone frequency to be employed, and the output of such oscillator is applied through the coupling condenser CI to the grid of the tube TI. The tube TI is normally non-conductive in its anodecathode circuit because of the negative bias of the battery 30 which is connected between the cathode'and the grid through the resistors RI and R2.

7 When it is desired to render the tube Tl of Fig. '7 conductive so as to a ply the frequency current generated by the oscillator to the modulator, and in turn to the transmitter. the bias battery 39 is shunted through the limiting resistor Rl to thus connect the cathode'to the grid through the resistor R2 and thus by-pass the negative grid bias normally provided by the battery 36. It is thus provided that the connection of ground to the lower terminal of the resistor R2, as is accomplished by the closure of front contact 3| of the control relay C of Fig.

'7 is effective to render the tube Tl conductive.

Similarly, wi h reference to Fig. 6. and also to the other drawings wherein tone generators are employed, the input wires to the respective tone generators when connected to, ground render the outputs of those tone generators effec ive to apply an input to the associated modulator, and those input wires can be assumed to be connected to respective terminals comparable to the lower terminal of the resistor R2 of Fig. '7.

It is therefore provided that a ground connection is closed during each ofi period of the oscillator IBUCT when the relay FF is picked up, to permit an output of the tone generator No. 3, and such output of the tone generator is a plied to the modulator, and after amplification the tone frequency current output of the modulator acts upon the transmitter F2 to modulate the pulse of the carrier wave which is radiated from the antenna A4 with the particular tone characterized by the tone generator No. 3.

Upon termination of the pulse by the dro ping away of relay FP, the openin of contact 29 term nates the modulation and the o ening of contact 21 terminates radiation of the carrier from the antenna A4. Because of the re ay FP having its circuit opened by the shifting of contact 26 to its lower position, the drop away time of relay FP as overned by the resistor H5 determines the duration of each pulse transmitted from field station No. 4.

In this hammer pulses are intermittently transmitted from the'end fie d Station No. 4 at the rate of the oscillator IBUCT under normal conditions with a single modulation applied to'each pulse by the tone generator No. 3 at th t station as a station call tone to call the next field station toward the control ofiice. Such next field station is illustrated in Fig. 5 as being intermediate field station No. 3, and although other field stations may receive the pulses transmitted from field station No. 4, only the field station No. 3 is responsive because of the particular station call modulation of these pulses.

Thus that which is received by the antenna A5 (see Fig. 5) is applied to the receiver F2 of Fig. 5, and the demodulated output of the receiver F2 is applied through the back contact 32 of relay 3RP3 to the input terminals of the respective tone filters Nos. 3, 4, 5, 6, '7,- 8 and 9. The filters Nos. 1 and 2 of this tone filter bank are not used ta-am 12 as the corresponding tones are used only as station call and station registration tones of a field station nearer to the control ofiice, and as a call for the control office itself.

Inasmuch as it has been described that only the tone No. 3 is normally applied to the pulses transmitted, there is an output of the tone filter No. 3 only, and such output is effective to energize the relay 3R For the purpose of simplification of the disclosure of this embodiment of the present invention, the circuits for the tone filters have not been shown in detail, but the organization of such filters can be according to the typical tone filter circuit which is illustrated in Fig. 8, or other types of tone filters may be employed in accordance with the requirements of practice.

With reference to Fig. 8, it is' illustrated that the output of a receiver which may be tuned to either the control or indication frequency Flor F2, as required, is fed through an amplifier'and applied through the coupling condenser C2 to the. input of a suitable trigger circuit organization and a suitable LC tank circuit is connected between the input of the trigger circuit and ground. The LC circuit comprises a condenser C3 and an inductive Ll connected in multiple, and this tank circuit by-passes the output of the amplifier to ground for frequencies other than the resonant frequency of the LC circuit, at which frequency the high reactance of the LC circuit permits sufficient current to flow in the trigger circuit to actuate such trigger circuit so as to provide an input for the gas tube T2 through a suitable coupling condenser C4.

The tube T2 can be a thyratron tube or a tube of this general character having the general characteristics of being non-conductive because of a negative grid bias supplied by a suitable negative bias battery 33 and adjustable grid resistor R3. The negative bias is so adjusted as to allow the tube T2 to be rendered conductive when there is an output of the trigger circuit associated therewith. Once the gas tube T2 is fired, the control relay R associated with that tone filter becomes energized in the anode-cathode circuit of the gas tube, and its energization is maintained until the gas tube becomes deionized. The deionization may take place in some cases by the opening of the anode-cathode circuit by a relay contact, and in other cases, the deionization of the gas tube is rendered effective by the use of alternating current for the power supply used in the anode-cathode circuit. In case alternating current is employed, the relayR in the anode-cathode circuit of the tube is maintained picked up only so long as the control grid of the gas tube is maintained sufiiciently positive to render the tube conductive. The relay R associated with the typical tone filter of'Fig. 8 is illustrated as having its control circuit energized by a suitable direct current B supply wherein (B-l-l is applied to the anode-cathode circuit through suitable contact selections which will be hereinafter considered for use in deionizing the associated gas tube. The cathode of the gas tube T2 is illustrated as being connected to both 3- and to CX, as a return circuit for either alternating current or direct current power supplies as may be employed for different tone filters.

Thus assuming that the tone filter No. 3 of as; tu e of the filter becomes ionized only in:

response to the output of the receiver of the tone No. 3 frequency. In such a casethe relay 3R is picked up in the anode-cathode circuit of the gas tube wherein BX is applied through back contact 34 of relay 3RP| and winding of relay 3R to the anode of the gas tube. The picking up of relay 3R by the closure of its stick contact 35 shunts the back contact 34 of relay tRPi out of thecircuit which has been described as the relay 3RP| is picked up in response to the picking up of relay 3R It is thus provided that the relay 3R is" pulsed at a rate corresponding to the rate of the pulses received by the antenna A5, such relay being maintained picked up for a period of time corresponding to the duration of eachpulse because of the use of alternating current for its energization through its associated gas tube whereby its energization is directly controlled in accordance with the positive energization of the control grid of the gas tube as a result of the reception of the pulse at the tone modulation No. 3.

Because of the pulsing of the relay 3R in response to the respective pulses received at station No. 3, the relay 8A which has slow drop away characteristics is maintained steadily picked up by the intermittent closure of front contact 35 of relay 3R It is thus provided that the relay 8A is maintained steadily p cked up, and thereby renders the oscillator 180C? normally inactive. It also selects that the pulse transmitted from field station No. 3 is measured by the dropping away time of relay 3RPI The relay 3RPI is picked up in. response to the picking up of the relay 3R at the beginning of each pulse received by the energization of a circuit extending from including front contact 31 of relay SA front contact 38 of relay 3R and winding of relay 3RPI, to (-l. The winding of the relay 3RPI is shunted by the resistor 39 and the halfwave rectifier 4! connected in series so as to render the relay 3RPI quick in picking up but slow in releasing in accordance with the desired mode of operation. It is well known that the drop away time is adjustable by varying the resistance of the resistor shunting the winding of the relay, and thus a proper resistance is selected for this resistor 39 to provide for a drop away time substantially comparable to the duration of the respective on periods which have been heretofore considered as being transmitted down at field stat on No. 4.

It is thus provided that in response to the dropping away of the relay SE at the end of each pulse received, the transmitter F2 is rendered active to transmit a carrier wave by the closure of front contact 4! of relay SRPl in series with back contact 15 of relay 3R The dropping away of relay 3R by the closure of its back contact 42 in series with the front contact 43 of relay SRPI renders the tone generator No. 2 effective to modulate the carrier wave transmitted from field station No. 3. The circuit extending from ground including front contact as of relay 5A back contact 42 of relay 3R and front contact 43 of relay 3R?! shunts the negative bias of the normally non-conductive gas tube associated with tone generator No. 2 which corresponds to the tube T! of Fig. 7, and thus renders tone generator No. 2 efiective to modulate the carrier wave transmitted by the the transmitter F2 at field station No. 3. It is thus provided that in response to the reception of each pulse transmitted from indication station No. 4 that is modulated by tone No. '3, a pulse is transmitted from field station No. 3

14 that is modulated with tone No. 2 indication station No. 2.

The field indication station No. 2 receives the pulses transmitted from indication station No. 3, such pulses being modulated according to the frequency of tone No. 2. The antenna A! at the field station No. 2 (see Fig. 4) is effective in a manner comparable with that described for field station No. 3 to relay the pulse through to the next station. The next station is assumed to be the control ofiice, and thus the station call tone No. 1 for calling the control office is applied to the pulse transmitted from field station No. 2.

To consider more specifically the mode of operation at the indication station No. 2 under normal conditions of the system, reference is made to Fig. 4 in which the output of the receiver F2 is fed through the back contact 45 of relay 2RP3 to the input terminals of the respective tone filters. Inasmuch as the input to the tone filters under normal conditions is only at the frequency of tone No. 2, tone filter No. 2 of Fig. 4 is the only one filter having an output. The relay 2R is energized for each pulse received by the antenna A! in the output circuit of the tone filter No. 2 by the energization of a pick up and a stick circuit comparable to that which has been described more specifically for the energization of the correspond ng relay 3R at indication field station N0; 3 (see Fig. 5).

The relays 8A and ZRPI are actuated in response to the relay llR in a manner comparable to that which has been described for similar relays at field station No. 3, and the oscillator CT is normally inactive as has been described for the corresponding oscillator at indication field station No. 3.

When the relay 2R. is dropped away at the end oi a pulse received from indication station No. 3, the tone generator No. l at that indication station is rendered effective to modulate the wave transmitted by the transmitter F2 which is rendered active at the end of the pulse upon the closure of front contact 45 of relay ZRPI in series with back contact 7! of relay 2R It is thus provided that a pulse modulated by tone generator N0. 1 is radiated from the transmitting antenna A8 of indication station No. 2 subsequent to the reception of each pulse by the antenna A! from the next distant indication field station. The ground connection shunting the negative bias which renders tone generator No. 1 effective to modulate the carrier wave transmitted extends from ground, includ ng front contact 47 of relay 3A back contact 48 of relay 2R and front contact 49 of relay ZRP! to the grid of the control tube used in governing tone generator No. 1.

At the control office the antenna A9 (see Fig. 3) receives the pulses radiated by the antenna A8 at the indication field station No. 2, and thus the demodulated output of the receiver F2 at the control ofiice is applied to the tone filters so that the tone No. 1 which is applied to the respective pulses transmitted from field station No. 2 triggers the gas tube associated with tone filter No. l and thus provides for the energization of the relay lR so long as the pulse is received. The lower winding of the relay IE is energized by alternating current so that deionization of the associated gas tube is rendered effective immediately upon the cessation of each pulse received by the antenna A9. The picking up of relay lR by the closure of front contact 59 applies (5+) to the windings of the respective relays 213. to SR inclusive so as to condition such relays so for calling thatthey may; be responsive to indication and station registration tones that may be received during that pulse.- It is considered at, present however that the normal conditions of the system exist wherein no indications are communicated, and therefore these relays are not responsive under the normal conditions bein considered.

Transmission of controls from the control oflice and 2 for the communication of controls.

The communication of controls is effective immediately upon the actuation of a start or send push button SPB preferably disposed upon the panel of the control machine, and thus no storage of designated controls is required, as all con'- trols are transmitted immediately upon the actuation of the associated start button. It is a matter of choice as to whether or not the button circuits are to be interlocked so that there can be no interference in case two buttons are simultaneously actuated, and according to Fig. 1 an interlock is provided between the buttons ZSPB and SSPB whereby the contacts of the relay 2-3C which repeat the button ZSPB are employed to segregate the circuits associated with the respective buttons so as to render the button ZSPB superior to the button 3SPB in case both buttons are simultaneously actuated. If this interlock is not required in practice, the respective wires connected to the center or common terminals of the various control switches for each field station can be broken through respective contacts of the associated start buttons SPB so as to be connected to ground when that button is actuated rather than using means such as the relay 23C to select transmission in accordance with whichever button is actuated.

Where there are more than two field stations in the system, and if an interlock is required in prevent interference between the start practice to buttons SPB when a plurality of buttons are simultaneously actuated, a selector system can be employed such as is shown, for example, in the prior application of Donald Blaisdell Ser. No. 758,833, filed July 3, 1947. According to such application a station selector unit is provided at the control office for governing the sequence of transmission of respective control cycles in accordance with a predetermined order of superiority so that for one complete cycle of operation of the selector unit, the control ofiice can transmit a control pulse, or cycle of pulses to each field station. It is also to be understood that a system for allowing transmission to but one field station at a time such as is disclosed in the patent to W. D. Hailes et al., No. 2,399,734, dated May 7, 1946, can be employed if desired.

With reference to Fig. 1 each of the tone generators illustrated is assumed to be of a structure such as is shown in Fig. '7, for example, wherein the connection of ground to the input wire of each of the respective tone generators renders that generator effective to modulate the associated transmitter Fl. It is thus provided that the connection of ground to the input wire of the tone generator No. 1 is efiective to modulate the carrier wave transmitted at the frequency F! for the transmission of a recheck control for the recheck of indications according to the tonechart of Fig. 9, and according to the general assign-'- ment of tones as has been heretofore described.

The particular field station for which the control is intended is identified by a distinctive station call tone assigned to that station, and according to the code chart of Fig. 9, tone No. 2 has been assigned to be used for calling the. field station No. 2 and the tone No. 3 has been assigned for calling field station No. 3. These tones are selected for transmission in accordance with the particular button SPB that is actuated. Thus, if the button ZSPB is actuated, tone No. 2 is transmitted for calling field station No. 2, and if the button 3SPB is actuated tone No. 3 is transmitted for calling field station No. 3.

More specifically, the actuation of the button 2SPB is efiective to pick up the relay 2--3C, and the picking up of that relay closes a circuit for rendering the control tube conductive of the tone generator No. 2. Thus by the connection of ground to the input control wire C2 through front contacts 5| and 52 of the relay 2-3C, the control tube governing the output of the tone generator N0. 2 is rendered conductive, and there is a tone generator output at the tone frequency No. 2 applied through the modulator to the transmitter Fl for modulation of the carrier wave radiated at the carrier frequency F! by the antenna Al. It will be noted that by the picking up of the relay 2--3C the opening of back contact 52 prevents the tone generator No. 3 from being rendered efiective to modulate the carrier wave in case the buttons ZSPB and 3SPB should inadvertently be actuated simultaneously.

It is similarly provided that if the field station No. 3 is to be called, the actuation of the button 3SPB is effective through back contact 52 of relay 2-3C to connect ground to the control wire C3 governing the control tube of the tone generator for the tone No. 3 so as to render the output of the tone generator effective to modulate the carrier wave radiated by the antenna Al.

If an operator of the control machine desires to have a recheck of the indications at any particular field station he actuates the recheck lever RCL for that field station to its right-hand position and then actuates the start button SPB for that field station. More specifically, assuming that it is desired to have a recheck of the indications at field station No. 2, the lever ZRCL is actuated to its right-hand position, and the button ZSPB is actuated. The relay 2-30 is picked up in response to the actuation of the button ZSPB, and upon the picking up of relay 2-30, ground is connected to the input control wire Ci of the tone generator No. 1 through front contact 5| of relay 2-3C, front contact 53 of relay 2-30, and contact 54 of lever ZRCL in itsright-hand position. Because of this circuit shunting the negative bias battery of the control tube for that tone generator, there is an output of tone generator No. 1 applied to the modulator, and the output of the modulator is eifective to modulate the wave transmitted by the transmitter Fl so long as the button 2SPB is maintained in its actuated position. r

In addition to the carrier wave being modulated by the output of the tone generator No. 1, and the station call tone, switch and signal control tonesare transmitted at the sametime to field station No. 2 in accordance with the positions of effective to modulate the carrier wave as s .ected the switch and signal control levers ZSML and ZSGL respectively. Thus, according to the tone chart of Fig. 9, with'the switch control lever 2SML in its left-hand position (normal switch position), the tone generator No. 4 is rendered effective to modulate the carrier wave by the connection of ground to the input wire C4 of that tone generator through front contact 5! of relay 23C, front contact 55 of relay 23C, and contact 55 of lever ZSML in its left-hand position.

Assuming the signal control lever ZSGL to be in its center position for designating a stop control for the signals at field station No. 2, tone generator No. 8 is rendered effective to modulate the carrier Wave of the transmitter Fl when the button ZSPB is depressed because of the connection of ground to the input wire C3 of the tone generator No. 8 through front contact 5| of relay 23C, front contact 5? of relay 23C, and contact 58 of lever ZSGL in its center position.

It is thus provided that upon the actuation of the start button ZSPB when the control levers for field station No. 2 are in positions as illustrated (except that the recheck lever ZRCL is actuated to its right-hand position as heretofore of the signal control lever ZSGL can select one :of three distinctive tones for transmission in accordance with its position. When the lever ZSGL is actuated to the right for clearing a signal I'governing trafiic to the right, the tone generator No. 6 is selected to be effective by connection of ground to wire C6 to modulate the carrier wave when the button 2SPB is subsequently actuated,

or the actuation of the lever ZSGL to its lefthand position is efiective to render tone generator No. '7 elfective by "connection of ground to wire i.

C! to modulate the carrier wave when the button ZSPB is actuated.

If the track switchat field station No. 2 is to be operated to its reverse position, the lever ZSML is actuated to its right-hand position, and thus 53 the contact 56 of that lever selects the tone generator No. 5 to be rendered effective to modulate the carrier wave radiated by the antenna Al.

If it is desired to call a maintainer at field "station No. 2, the maintainers call lever ZMCL is actuated to its right-hand position, and by such actuation the closure of contact 59 of that lever selects the tone generator No. 9 to have its output modulate the carrier wave radiated from the control oifice upon actuation of the button ZSPB.

It will be readily apparent that a similar mode of operation is effective for the transmission of controls'to field station No. 3 in accordance with selections by the respective levers BEE-CL, 3MCL, '3SML and SSGL upon actuation of the associated button BSPB. Corresponding generatorsare rendered effective to modulate thecarrier wave for therespectivepositions 'oi'the various level tone gen'eratorsthat have been described as l.

o'f' -the station call tone 5N0. 3 ..:for transmission rather than the station call tone No. 2 as has been described for transmission to field station No. 2. The circuit that has been described whereby the tone generator No. 2 was rendered efiective to modulate the carrier wave for transmission to field station No. 2 is opened at front contact 5| of the relay 23C, when transmitting to field station No. 3, and the closure of back contact 52 connects ground to the control tube of tone generator No. 3 through the button 3SPB. Similarly the selective connection of ground to the control tubes of the respective tone generators is made effective when the button SSPB is actuated through the respective back contacts 53, 60, 55 and 5? of the relay 2-3C. The front contacts 53, 6t, 55 and 5? which have been described as being used in the circuits for transmission to field station No. 2 are open and thus the control of the tone generator is non-responsive tothe positions of the respective levers provided for field station No. 2 when the button 3SPB is actuated for transmission to field station No. 3.

Control of devices at field station.

With reference to Fig. 2, that which is transmitted from the control office is received by the antennas A2 and A3 at the respective field stations Nos. 2 and 3 as both of these field stations are assumed to be within range of the transmitter Fl at the control office. The receiver Fl each of the field stations is tuned to the carner frequency transmitted from the control office, and the output of each of the receivers Fl is applied to the bank of tone filters associated therewith. The output of the respective tone filters of each filter bank is of course dependent upon the tones received as modulations of the carrier wave radiated from the control office. Thus the modulated carrier wave is demodulated by each of the receivers Fl, and. the output of such receivers is applied to their associatedtone filter banks as demodulated tones which can be at audio-frequency substantially corresponding to those used for modulation of the carrier wave at the control clfice, although it is to be understood that higher frequencies can be employed in accordance with the requirements of practice as been heretofore pointed out.

As a matter of convenience in illustrating the manner in which respective tones are used for various purposes, the numbering of the tone filters has been made to correspond with the numbering of the tone generators at the control elites and. thus the tone filter No. 1 is responsive to the modulation of the carrier wav by tone generator No. 1 at the control office. tone filter No. 2 is responsive to the modulation of the carrier wave by tone generator No.2, etc.

If there is an output of tone filter No. 2, at field station No. 2. the station relay 280 which is connected in the output circuit of that tone filter is picked up in response to the station call of field station No. 2. The relay 2S0 is energized in such output circuit by rectified alternating current because of its connection to the anode of the output gas tub-etc which alternating curnal of the alternating current" supply (351) is co. rooted to the winding of the station relay and the other terminal of the alternating current supply (CX) is connected to ground and .is to the cathode of the'gas tube for that tone er as shown in Fig. 8. If the station call nsniitted from the control ofiice is forstation irus creating modulations cr lpne n'cse. atheithan tone No. 2, there is no station relay picked up at field station No. 2 but the station relay 3S0 at field station No. 3 is picked up because of the output of the tone filter No. 3. It is thus provided that each field station has a station relay responsive to its station call tone, and such station relay is non-responsive to the station call tones of other stations.

is thus provided that the picking up of the relay 230 in response to the call of station No. 2 from the control ofiice is effective by the closure of front contact 65 to condition the recheck relay ZRC, the switch control relay 2WZ, the signal control relay ZLGZ, ZRGZ and 2B, and the maintainers call relay ZlViC so that such relays can be responsive to their associated control tones that are applied to that particular control pulse.

When the relay 230 at field station No. 2 is picked up upon the reception of a control puise, the relay 2R0 can be picked up through front contact SI of relay 2S0 if tone No. l is applied to that pulse for a recheck of indications from the field station No. 2. If no recheck is called for, there is of course no tone No. 1 transmitted and thus the relay 2R0 is non-responsive during such pulse.

The magnetic stick relay 2WZ is distinctively responsive to the reception of tone No. 4 or tone No. 5 of a control pulse transmitted for field station No. 2. When the tone No. 4 is received on the control pulse, the relay 2W2 is picked u to govern the operation of the track switch ESW to its normal position. If the tone No. 5 is received, the relay ZWZ is dropped away and the track switch 28W is operated to its reverse position. Although the details of the circuits have not been shown for the power operation of the track switch ZSW by the switch machine 28M, such circuits have been indicated as being governed by the contacts 62 and 63 of the relay ZWZ, and the manner in which the switch 4 machine 28M is controlled by such contacts is well known to those skilled in the art.

ihe signal control relays ERGZ and ZLGZ, rather than being of the magnetic-stick type, are preferably provided with stick circuits as illustrated whereby either of such relays when picked up is maintained picked up dependent upon passage of a train. The relay ZRGZ is picked up upon reception of tone No. 6 in response to an output of tone filter No. 6 by the energization of its upper winding through back contact 54 of relay ZLGZ and front contact 6! of the station relay 28C. Upon the picking up of this relay, the closure of front contact 65 permits the clearing of signal ELA or signal 2LB as selected by the position of the track switch 23W. A stick circuit is established for the lower winding of the relay ERGZ upon the picking up of that relay extending from including front contact 65 of the track relay 2TB, back contact ii! of the relay 23, front contacts 88 of relay ZRGZ, and lower winding of relay ZRGZ, to

Similarly if the control pulse is modulated with tone N0. 7, the relay. ZLGZ is picked up for the clearing of signal ZRAor'gZRB as selected by the position of the track switch ZSW. The relay ZLGZ can be picked up by the output circuit of tone filter No. 7 which includes the upper winding of relay ELGZ, back contact 69 of relay ZRGZ, and front contact 6! of relay 280. The closure of front contact 'i'il of relay 2LGZ is effective to permit the clearing of signal ZRA or signal ZRB. A stick circuit is established for the lower winding of relay ELGZ upon the picking up of that relay through front contact 66 of relay 2TB, back contact 61 of relay 2B and front contact ll of relay ZLGZ for governing the restoration to stop of the signal in response to the passage of a train, or in response to the communication of a stop control from the control office. Signal stop control can be transmitted from the control office in a manner which has been described whereby the tone No. 8 is used for modulation of the control pulse for station No. 2, and the relay 2B is energized in the output circuit of tone filter No. 8 through front contact 6! of the station relay 28C.

If a maintainers call is communicated from the control office to field station No. 2, the control pulse is modulated by tone No. 9, and the output of tone filter No. 9 is effective to pick up the maintainers call relay ZMC by the energization of the upper winding of that relay through front contact 55 of the station relay 25C. The relay ZMC when picked up energizes the maintainers call indicator lamp ZMK upon the closure of front contact 12, and a stick circuit is closed through front contact 13 of relay 2MC to maintain the lamp ZMK illuminated until the maintainers cancel button ZMPB is actuated to open such stick circuit.

Having thus described the manner in which the relays at field station No. 2 are responsive to control tones communicated from the control office, it is to be understood that such an organization of tone responsive relays is provided at each field station, and that the response of the control or application relays at each field station is dependent upon the station relay for that station as the relays at field station No. 2 are dependent upon the closure of front contact 6| of the station relay ZSC. The application relays at field station No. '3, for example, are dependent upon the picking up of the relay 38C to close front contact i i of that relay, but otherwise are governed the sameas the corresponding relays at field station No. 2.

Transmission of indications from end field station It has been described when considering the normal conditions of the system that the end field station No. 4 transmits so called permission pulses intermittently at a per minute rate to field station No. 3, such pulses being modulated only by the tone No. 3 which is the station call tone for the next field station nearer to the control ofiice. If a change takes place in one of the devices at field station No. 4 which is to be indicated at the control oflice, the next pulse subsequent to such change is modulated by selected tones in addition to the tone No. 3 for the communication of indications as to the new position of the device in which the change has occurred. In other words, the change in a device initiates a transmission of the indication of the new position of that device, but this is the only indication that is transmitted, as compared to transmitting the conditions of all of the devices at field station No. 4 each time that a change occurs.

To consider a typical condition for the transmission of an indication from field station No. 4;, it will be assumed that a signal has'been -cleared nation of tone generators rendered effective t modulate the next pulse to by the dropping away dicating a signal clear for at field station No. i in accordance with controls communicated from the control office for governing traiiic to the right; and in accordance with the clearing of this signal, in a manner Well known to those skilled in the art, the signal stop "repeater relay RM is dropped away, and the dropping away of that relay by the shifting of contact I? interrupts the stick circuit for the change relay 56H to cause that relay to be dropped away.

By the dropping away of the relay I CH the relay SCI-1P is dropped away because of the opening of its pick-up and stick circuits at front contacts 80 and 22 respectively. Upon the dropping away of relay lCHP the opening of its front contact 25'opens the pick-up circuit for the relay I to cause that relay to be dropped away and to connect ground to respective control circuits selected by the relay QCHF for selecting a combito have their outputs be radiated by the antenna A". Inasmuch as the change may occur during the transmission of a pulse, it is desirable rather than to modulate the remainin portion of that pulse according to the indication code, that the transmission of the indication be delayed until that pulse is terminated.

It is therefore provided that the relay I is maintained picked up by a stick circuit extending from including front contact ti of relay FF and front contact 82 of relay 1 so as to prevent the dropping away of 6 and 8 are simultaneously transmitted during the transmission of anindication pulse foringoverning traffic to the right. tones, the tone No. 3 is applied to the pulse as has'be'en described for calling the station No.

andinaddition the tone No. i is applied to the pulse as the registration tone for the field station transmitting the indication so'that when such tones are-relayed through to the control of- ".fice, to identify the particular field station that is transmitting the indications.

t is therefore provided that the tone generator No. 3 isrendered effective to modulate the pulse radiated by the antenna A l in a manner which has been described for the purpose of calling the next field station nearer to the control oiiice; the output of tone generator No. i is rendered effective in accordance with the dropping away of the relay I to modulate the indication. pulse radiated by the antenna A l by (1115 connection of ground to shunt the negative bias of the control tube (not shown) associated with that tone generator through contact 28 .of oscillator 553 50? in its lower-position, front contact as of relay F? and back contact 83 of relay 1*; the output oftone generator No. 5is rendered eiiective modulate the indication pulse radiated the antenna Ad by reason of relay lCI-IP being dropped away to connect ground to the control tube associated with that tone generator through contact 23 of oscillator 435C? in its lower position, front contact '29 In addition'to transmitting these 28 of oscillator HiCCT in its lower position,

from contact 2% of relayFP, back contact'83 of relay 2*, back contact 85 of relay iCHPfi and back contact 3"! of relay RMK During the transmission of the indication 5 pulse, the relay ECH -xvhich has initiated the transmission of the indication is restoredto-its picked up position by a pick-up circuit extending from includingcontact 26 of oscillator Kid-GT in its lower position, front contact 83 2Q of-relay PP, back contact in) of relay Iiback contact fill of lCHP winding of relay lCH iront contact l5 of relay Llvi back contactll r'elay PtM and back contact E2 of the recheck control relay RC to It will be noted that this pick-up circuit is closed only for a short time (corresponding to the duration of the indication pulse transmitte'dirom field'station No. 4), such time being determined the dropping away time of the relay FP. It

30 is thus desirable that the relay F? be sufliciently slow in dropping away to permit the picking up of the relay i63 after the circuit for "the re ay F? has been opened by the shifting of oscillator contact to its lower position. The

other consideration in accordance with the'timingof relay FF is thatit must be made sufficiently slow in dropping away to provide the desired duration of each indication pulse that transmitted from the field station No. 4.

0 After the relay [CH has been restored to its =3 cluding contact of oscillator contact picked up positiomit is maintained picked up by its stick circuit which has been heretofore described.

After the'relay'l P has become dropped away 45 to terminate the pulse transmitted from field station No.4, the closure of its back contact 83 closes alpiclnup circuitforthe change repeater re a'- lCHP to provide for the'restoration of that relay. This circuit extends from in- !850'1 its lower position, back contact 8% of relay FP, halfvfai e rectifier @Liront contact 86 of relay 'ICH 'and'winding or relay iCHPfl to The picking up of this relay, bythe closure of its front establishes a stick circuit tomeintain the relay -5CHP steadily'picked up, dependent upon the change relay CH2 It 'Willbe noted that h'e shifting of contact 92'? upon the picking up of relay iCi-IP provides thattheirelay ICI-I 0 will not haveits pick-up circuit energized during subsequent on periods of transmission, and thus will subsequently be at all times subjeit to dropping avvay 'as'a result of the opening of its stick circuit.

After the relay ICHP is picked up, assuming there are no other relays CH1? dropped away for the transmission of other indications, a pick-up circuit is closed for the indication control relay 1 upon the closure of front contact-25 of relay lCHP and the picking up of that relay is efiective by the opening of back cont ct llfl to prevent the modulation of the next oifperiod carrier'pulse t at is transmitted.

In a similar manner indication pulses relative ti: "the condition 61" the track Switbh Bit caress 23 tion No. 4 are transmitted in accordance with the positioning of the switch correspondence repeater relays NWC and RWC which are used to repeat the respective normal and reverse correspondence conditions of the track switch at field station No. 4, such relays being controlled in a manner well known to those skilled in the art. Any change in the position of either of such relays is effective to interrupt the stick circuit of the change relay. 20H in a manner comparable to that which has been specifically described for the interruption of the circuit for the relay ICH by the relay RM? The relay 2CHP is dropped away by the opening of its stick and pick-up circuits at front contacts 2! and 93 respectively of relay 2CH and upon the dropping away of relay 2CHP the opening of front contact 24 is effective to cause the dropping away of the indication control relay 1 to render the outputs of the respective tone generators eiiective to modulate the next pulse as an indication pulse with tones selected according to the contacts 94 and 95 of the relays NWC and RWC respectively and the back contacts 96, 9? and 98 of relay 2CI-IP The particular tones selected for modulation of the indication pulse correspond with the tone chart of Fig. 16 wherein the transmission of tones 5, i and 8 is an indication of the switch being locked and is transmitted when either front contact 95 of relay RWC or front contact 94 of relay NW is closed. If both of these contacts are open, the tones 5, 1 and 9 are transmitted so as to distinctively indicate at the control office that the track switch is unlocked and/or out of correspondence.

The modulation of the indication pulse under consideration by the output of tone generators Nos. 3 and 4 is rendered effective in a manner which has been described, and the transmission of tones 5 and i during that pulse is rendered effective in accordance with the connection of ground to shunt the negative bias of the control tubes (not shown) associated with the respective tone generators Nos. 5 and 7. The ground connection for shunting the negative bias of the control tube (not shown) associated with tone generator No. 5 includes contact 28 of oscillator l80CT in its lower position front contact 29 of relay FP, back contact 83 of relay 1*, front contact 84 of relay ICHP, and back contact 96 of relay 2CHP In a similar manner the shunting of the grid bias of the control tube associated with tone generator No. '7 is accomplished by the connection of ground through contact 28 of oscillator |89CT in its lower position, front contact 29 of relay FP, back contact 83 of relay 1, front contact 85 of relay ICHP and back contact ,9! of relay ZCHP. The tones Nos. 8 or 9 are selected so that the negative bias of the control tube (not shown) associated with tone generator No. 8 is shunted by the connection of ground through contact 28 of oscillator I89CT in its lower position, front contact 29 of relay FP, back contact 83 of relay 1 front contact 86 of relay ICHP back contact 98 of relay ZCHP, and front contact 95 of relay RWC (ii relay RWC is picked up) or back contact 95 of relay RWC and front contact 94 of relay NWC (if relay NWC is picked up). -In case the relays NWC and RWC are both dropped away, the negative bias of the control tube (not shown) associated with the tone generator No. 9 is shunted by the ground connection including contact 28 of the oscillator l8llCT in its lower position, front contact 29 of relay FP. back contact 24 83 of relay 1*, front contact 88 of relay iCI-IP. back contact 98 of relay ZCHP, back contact 95 of relay RWC and back contact 94 of relay NWC The restoration of the relay 2CH during the transmission of the indication pulse is accomplished in a manner comparable to the restoration of the relay lCH the pick-up circuit for relay 2CH extending through front contact 90 of relay ICHP so as to establish a chain circuit system of priority whereby the relays CH are restored one at a time, that is, one after each indication pulse, in the order according to their positions in the chain circuit and in numerical order as indicated by the preceding numerals of the reference characters used for designation of these change relays. More specifically, if the reiays [CH and 261-1 are both dropped away at the same time, the indications associated with the relay ECH will be transmitted during the first pulse, and the relay lCI-I will be picked up during that pulse, but because the relay ICHP' cannot be picked up until the relay FF is dropped away to terminate the pulse that is being transmitted and open the pick-up circuits for the relays CH at front contact 89.

The relays 301-1 and 3CHP are associated with the communication of track indications to the control office from field station No. 4 as selected by the track relay TBA It is thus provided that the shifting of contact i i of {he track relay TR in the stick circuit for the change relay 301-1 is efiective to initiate the transmission of a track indication. The back contacts 99, I60 and iii! of the relay SCI-1P are efiective to select tones 6, 'i and 8 for transmission if the track is unoccupied in accordance with the closure or" front contact 02 of the track relay TR, or the tones 6, l and 9 if the track is occupied as selected by the closure of the back contact I92 of relay TR In case it is desired to have a recheck of all of the indications at the field station No. 4, a recheck control is transmitted from the control oifice in a manner corresponding to the mode of operation of the system for the transmission of recheck controls as it has been heretofore described. The transmission of such a control to field station No. 4 provides for the momentary picking up of the recheck relay RC and the picking up of that relay, by the opening of back contact 52, interrupts the stick circuits for all of the relays CH at field station No. 4, and thus provides that all of the relays CH at field station No. 4 are dropped away. As aresult of the dropping away of the relays CH the relays CHP are all dropped away, and the dropping away of the relay I conditions the system for the transmission of the first indication pulse so that with the relay FP picked up the shifting of contact 29 of the oscillator [890T to its lower position provides for the transmission of indications as selected by the positions of the signal repeater relays LM4 and RM which are associated with the relay ICHP It will be noted that the relay I is delayed in dropping away by its stick circuit including front contact 593 of relay RC until the relay RC is restored to its normal dropped away position; thus insuring that there will be only one recheck oi the indications irrespective of how long the recheck button at the control ofiice is maintained closed. 7

During the transmission of the signal indication pulse, the relay ICH is restored and upon the dropping away of relay FP at the end of the pulse, the-relay lCHPt-is icked ups-thus closing circuits at front contacts 84, 8 and 85 to condi:

third pulse is used for the transmission of track indications and the pulse is modulated according to selections by the contact l 82 of the track relay PR and the back contacts 99, it?) and lid of the relay SCI-1P The relays 301-1 and QCHE are restored during and at the end of the track indication pulse respectively, and the picking up of relay 3CHP closes a circuit which has been described for the picking up of the relay 1* to thus restore the conditions of the indication transmitting apparatus after a complete recheck oi the indications is accomplished.

If it were possible, for example, when the tracl, indication pulse were being transmitted, for the relays 2CH and ZCHP to be dropped away, the opening of front contacts 96, 9? and 98 f relay ZCHB would-interrupt the circuits for selecting the tonestransrnitted for track indication, and change thecombination of tones transmitted in accordance with selections for the transmissionof-switch indication. Obviously this mode of operation would be undesirable, and it is therefore provided that Whenever the transmission of indications has been initiated by the dropping away of the-relay 1*, all r lays CHP which remain picked up, are-maintained energized-by their stick circuits as long as the relay I is dropped away. Thus the relay {CW is maintainedpicked up by the closure of back contact H34 cf-relay I relay 2CHP is maintained picked up'by back contact I55 of relay 1 and relay 3CHP 'ismaintainedpicked up by back contact H36 of relay 1. It istherefore provided that although any change relay CH may be dropped away at anytirne, the associated relay CH? will be maintained pickedup until indications which have beeninitiated by prior starts are communicated. When such communication is complete, the relay I is picked up, and by the picking up of such relay, all relaysCI-IP are allowed to drop awaythat have their associated change relay CH- d oppe w it Rel yof indications at intermediate station Having. thus considered the manner in which indications are transmittedfrom field station No. 4, consideration will now be given as to them'odev of, operation at the intermediate field stationNo. 3 forthe reception of the indications transmitted fromfield station No. 4, and the relaying of such indications from station to station to accomplish the transmission of indications from the most distant fieldstation to the controloflice.

Uponconsidering the transmission of indications from field station No. 4, it has been pointed out that each indication pulse transmitted from that-field station is modulated by tone No. 4 as the station registration tonev for the field station transmitting the indications, and indication tones are selectively appliedto the pulses in accordance witlrthe indications to be communicated. These tonesareall in additionto the tone No. 3 which is normally transmitted from the field station No. 4asastation call of the next fieldstation toward the controlofficeyas has been speci 'ially considered when describing the normal condi tions of the system.

Thus with reference to Fig. 5, the reception ol' an indication pulse by the receiver F2 applies a plurality of tones simultaneously to the respective tone filters associated therewith in accordance with the tones used for modu n of the carrier Wave transmitted from field station No. 4. It is provided that the relay 3& is picked up in a manner which has been described by its energization in the output circuit of the tone filter No. 3, and the closure of front contact iiii of that relay conditions the relays R for higher tone numbers so that they can be picked iip, in series with the relay in accordance with the reception of their associated tones. It is therefore provided that because of the reception of tone No. 4,

with relay 3R picked up, the relay 3 is pickedup in the output circuit of tone filter No. 4. The circuit for relay 4R extends from (5+) and includes front contact to! of relays 3R winding terstics of the relay ERPI as provided by the resistance 39 shunting the winding of that relay in series withlthe half-wave rectifier unit 39. It is thus provided that each of the relays for tones higher than tone No. 3 at field station No. 3, together with the relay Y is maintained picked up for aperiod oi time subsequent to the termination or the pulse received, even though the relay SR is dropped away immediately upon termination of the pulse received because of its energizetion rectified alternating current in the anodecathode circuit of thegas tube associated with the tone filter No. 3. In addition to the picking up of the relay 4R upon the reception of an indication pulse from field station No. 4, the relays 5R ER ,*'ER SR and QR are selectively governed in response to the particular indication tones received.

Because of the relay Y being picked up in response to the reception or" an indication pulse, the relay SRPZ is picI-zedup as a repeater of relay 3R3 by the energization of a circuit extending from including front contact 3? of relay SA front contact 38 of relay 3R winding of relay 3RP2, and front contact I it of relay Y to In response to the picking up of the relay BRPE, the closure of front contact i l l causes the pick up of the repeater relay SRPB. Upon the picking up of the relay SRPB the back contact 32 inthe output circuit of the receiver F2 is opened to remove tone energyrfrom the tone filters for a substantial period of time to prevent the possibility of the tone filters having their input circuit erroneously energized by tones transmitted from some intermediate station nearer the control office and within transmitting range of field station No. 3. The relay ERPB is preierablymade Substantially slow in dropping away by the resistor l l2 in series with the half-wave rectifier l i3 shunted across the winding of that relay so as to be maintained picked up duringthe relaying of theparticularindication pulse under consideration through the next field station toward the control office.

Although it makes little difference whether or not the pulse received is terminated. prior to the opening Oi' the output circuit of-the receiver F2 at field station No. 3 by the back contact 32 of relay SRPS, it will be assumed that the opening of this circuit terminates the pulse as far as the 

